refactor: Extract message_feed_loading module from message_scroll.

This new module allows us to remove dependency on `message_scroll` in `fetch_status`
and hence allowing us to migrate it to TypeScript.

web/src/message_feed_loading.ts

@@ -0,0 +1,41 @@
+import $ from "jquery";
+
+import * as loading from "./loading";
+
+let loading_older_messages_indicator_showing = false;
+let loading_newer_messages_indicator_showing = false;
+
+export function show_loading_older(): void {
+    if (!loading_older_messages_indicator_showing) {
+        loading.make_indicator($("#loading_older_messages_indicator"), {abs_positioned: true});
+        loading_older_messages_indicator_showing = true;
+    }
+}
+
+export function hide_loading_older(): void {
+    if (loading_older_messages_indicator_showing) {
+        loading.destroy_indicator($("#loading_older_messages_indicator"));
+        loading_older_messages_indicator_showing = false;
+    }
+}
+
+export function show_loading_newer(): void {
+    if (!loading_newer_messages_indicator_showing) {
+        $(".bottom-messages-logo").show();
+        loading.make_indicator($("#loading_newer_messages_indicator"), {abs_positioned: true});
+        loading_newer_messages_indicator_showing = true;
+    }
+}
+
+export function hide_loading_newer(): void {
+    if (loading_newer_messages_indicator_showing) {
+        $(".bottom-messages-logo").hide();
+        loading.destroy_indicator($("#loading_newer_messages_indicator"));
+        loading_newer_messages_indicator_showing = false;
+    }
+}
+
+export function hide_indicators(): void {
+    hide_loading_older();
+    hide_loading_newer();
+}